A. Field of the Invention
The present invention relates to a method of forming a corner portion of a torque converter by a press working.
B. Description of the Background Art
A torque converter is a device which has three kinds of vane wheel members, an impeller, a turbine and a stator, which work together for transmitting a torque via movement of a working fluid. The impeller is fixed to a front cover coupled to a rotary member at an engine side, and transmits the torque to a transmission through the working fluid flowing from the impeller toward the turbine.
The front cover which is a component of the torque converter is formed by a press working in many cases. The front cover generally has a disk shape with a cylindrical shaped portion formed at a radial outer periphery thereof. The front cover is formed from a single plate material, pressed into the above described shape. An annular corner having a curved contour in cross-section is therefore formed between the disk shaped portion and the cylindrical shaped portion. The front cover 3 has a plate thickness of 4-6 mm, and the annular corner has a curvature radius nearly equal to the plate thickness.
An impeller shell of the impeller, which is another component of the torque converter, is formed by the press working in many cases. The impeller shell usually has a plate thickness of 3-5 mm and is provided at its radially inner portion with a curved portion having a curvature radius of 5-6 mm.
The press working which is performed on the front cover having the above form generally reduces the thickness of the radially outer curved portion. For suppressing this reduction in thickness, the front cover is usually formed stepwise, and more specifically a preliminary press working is effected before a finish press working. Under present circumstances, however, the thickness reduces 7% or more from the thickness of the blank material used to form these members. For avoiding reduction in strength due to the pressing operations, thick plates have been used as blanks in the prior art. Such a thick blank increases a production cost and weight of the subsequently produced torque converter.
In the impeller shell having the foregoing form and formed by a press working, a thickness is likewise reduced at the radially inner curved portion. More specifically, the thickness of the blank decreases at a rate from 5 to 20% when finished.
The impeller shell forms a working fluid chamber of the torque converter together with the front cover, and therefore is subjected to an internal pressure by the working fluid. If a thin portion is formed at the impeller shell, the torque converter may be deformed such that the dimension of the torque converter expand, which may result in disadvantages such as breakage and disengagement of parts. In order to avoid these disadvantages due to reduction in strength and rigidity caused by reduction in plate thickness, thick blanks have been employed in the prior art. However, such a thick blank increases a production cost as well as a weight of the torque converter.